1. Field of the Invention
The present invention relates to a printing apparatus that creates dots on a printing medium during a main scan and thereby prints an image. More specifically, the present invention pertains to a printing apparatus that is capable of adjusting the dot creation timings during the main scan.
2. Discussion of the Background
An ink jet printer is one of the printing apparatuses that perform a main scan and sub-scan of a print head and prints a multi-color image. The ink jet printer ejects inks of multiple colors, for example, cyan, magenta, yellow, and black to create dots. Creation of dots with the multiple color inks at various recording ratios results in printing a multi-color image. To attain the high quality printing by the ink jet printer, it is desirable that there is no relative misalignment of the positions of dots created with the respective color inks. For the purpose of preventing such misalignment, the timings of dot creation with the respective color inks are adjusted at the time the ink jet printer is shipped.
Some of the ink jet printers create dots in both a forward pass and a backward pass in a main scanning direction to enhance the recording speed (such recording technique is hereinafter referred to as the bidirectional recording). To print an image of favorable quality, it is necessary to make the dots formed in the forward pass align with the dots formed in the backward pass in the main scanning direction.
FIGS. 15(a) and 15(b) show states of dots created by the bidirectional recording technique. Open circles represent dots formed in the forward pass of the main scan, whereas closed circles represent dots formed in the backward pass of the main scan. FIG. 15(a) shows a state, in which the dots formed in the forward pass align with the dots formed in the backward pass in the main scanning direction, FIG. 15(b) shows another state, in which the dots formed in the backward pass are shifted rightwards relative to the dots formed in the forward pass. The relative misalignment of the dots formed in the forward pass with the dots formed in the backward pass causes an unevenness of density and thereby lowers the picture quality of the resulting printed image.
The misalignment of the positions of dot creation in the forward pass and in the backward pass is caused by a diversity of factors, such as plays (backlashes) required in the driving mechanism of the printer. The misalignment is also due to a variation in thickness of the printing medium or printing paper.
FIGS. 21(a) and 21(b) show misalignment of the positions of dot creation in the forward pass and in the backward pass according to the thickness of printing paper. In the example of FIG. 21(a), a dot dt11 is formed on a sheet of printing paper PA1 in the forward pass, and a dot dt12 is formed adjacent to the dot dt11 in the backward pass. A nozzle Nz ejects ink droplets Ik11 and Ik12 at respective positions shown in FIG. 21(a), which are determined by taking into account the speed of the forward pass and the backward pass. The ink droplets Ik11 and Ik12 respectively draw loci shown in FIG. 21(a) and hit target positions to form the dots dt11 and dt12.
FIG. 21(b) shows a state with a sheet of thicker printing paper PA2. In this case, the distance between the nozzle Nz and the printing paper PA2 is less than the distance between the nozzle Nz and the printing paper PA1 in the example of FIG. 21(a). Ejection of ink droplets in the forward pass and in the backward pass at the same timings as those in the case of FIG. 21(a) causes ink droplets Ik21 and Ik22 to respectively draw loci shown in FIG. 21(b) and hit against the printing paper thereby forming dots dt21 and dt22. Accordingly, there is an undesirable gap between the dots thus created, and the resulting recorded image is different from a target image to be recorded. In order to obtain the target image to be recorded, the timing of dot creation in the backward pass should be set later than the timing shown in FIG. 21(b).
The conventionally adopted technique adjusts the dot creation timing using a test pattern so as to prevent the misalignment due to the diversity of factors. The technique records a predetermined test pattern while varying the dot creation timing in the forward pass and in the backward pass. The dot creation timing is then adjusted to the timing that gives the favorable results of recording. By taking into account the diversity of factors discussed above, the adjustment of the dot creation timing should be performed not only when the printer is shipped but also in occasions required by the user.
The prior art technique actually performs the adjustment of the dot creation timing only for one color, that is, the black ink, and collectively modifies the dot creation timings of the other colors based on the results of the adjustment.
The adjustment of the dot creation timing is not performed sufficiently in conventional printers. The insufficient adjustment causes the originally low picture quality of the resulting printed image in some printers and lowers the picture quality with an elapse of time in other printers. In the printer of the bidirectional recording, the dot creation timing is adjusted according to a test pattern. Such adjustment may, however, not sufficiently improve the picture quality of the resulting printed image. The deteriorating picture quality is partly due to a misalignment of dot recording positions between different colors.
The deteriorating picture quality due to the misalignment of dots is found not only in the case of bidirectional printing but in the case of performing printing operations only in a single direction of the main scan (hereinafter referred to as the unidirectional recording). The print head in a printer typically has a large number of nozzles that are arrayed in both the main scanning direction and the sub-scanning direction to have a two-directional arrangement. Unless the dot creation timing is adequately adjusted between nozzles having different positions in the main scanning direction, there is a misalignment of dots in the main scanning direction in the case of unidirectional printing. In the printer having a plurality of different color inks, there is a variation in ink ejection speed due to the difference in characteristics of the respective inks. This also leads to a misalignment of dot recording positions. A variation in ink ejection properties due to the difference in driving mechanism of nozzles also results in a misalignment of dot recording positions. Such misalignment lowers the picture quality of the resulting printed image.
The recent trend requires the printer to record fine dots and enable printing with a high resolution. In the case of printing with a high resolution, however, only a slight misalignment of dots may correspond to a misalignment of dot recording positions by the unit of pixels. In the printer that performs printing with a high resolution to improve the picture quality, the deteriorating picture quality due to such misalignment is not negligible. Not only the misalignment of dot recording positions between different colors, but any misalignment of dots is not of course negligible for the improvement in picture quality.
Accordingly, an object of the present invention is to solve the problems discussed above and to prevent a positional misalignment of dots and attain high-quality printing in a printing apparatus that performs a main scan of a print head to print a multi-color image.
To achieve these and other objects, the present invention provides a printing apparatus that performs a main scan and causes a print head having nozzles for ejecting ink to create dots on surface of a printing medium at a predetermined dot creation timing during the main scan. The main scan moves the print head forward and backward relative to the printing medium. The print head has a plurality of nozzle groups, each nozzle group including a plurality of nozzles having a predetermined common condition relating to ink ejection. The printing apparatus includes a timing specification unit that inputs an instruction to change the dot creation timing in the course of the main scan with regard to each of at least two nozzle groups selected among the plurality of nozzle groups, an adjustment unit that adjusts the dot creation timing with regard to the each nozzle group, based on the input, and a drive control unit that drives each nozzle group at the adjusted dot creation timing in the course of the main scan so as to create dots.
A variety of settings may be applicable to specify the nozzle groups according to the structure of the printing apparatus.
In accordance with a first setting, in the case where the print head is capable of ejecting inks of multiple colors, each of the plurality of nozzle groups includes a plurality of nozzles that eject an identical color ink.
It is not necessary to change the dot creation timing individually with regard to all the nozzle groups provided on the print head. For example, in the first setting, the at least two nozzle groups, which are objects of the changing instruction, may correspond to specific colors that are selected out of the multiple colors and other than a predetermined color having little effects on picture quality.
In accordance with a second setting, each of the plurality of nozzle groups includes a plurality of nozzles having an identical position in a main scanning direction.
In accordance with a third setting, in the case where the print head has a sufficient number of driving units (each driving unit having a plurality of driving elements for driving the nozzles so as to enable one driving element to be mapped to one nozzle), each of the plurality of nozzle groups includes a plurality of nozzles that are driven by an identical driving unit.
In accordance with a fourth setting, in the case where the print head ejects a plurality of different inks having different properties relating to ink ejection, each of the plurality of nozzle groups includes a plurality of nozzles that eject ink having a practically identical property. The property relating to ink ejection is, for example, viscosity, specific gravity, or surface tension of ink.
In the fourth setting, when the print head ejects a plurality of different inks having different densities, each of the plurality of nozzle groups includes a plurality of nozzles that eject ink of an equivalent density. For example, in the case where the print head has both a higher density ink and a lower density ink for cyan and magenta, nozzles corresponding to the cyan and magenta inks of the higher density are included in one nozzle group, whereas nozzles corresponding to the cyan and magenta inks of the lower density are included in another nozzle group. Such setting is applicable to the print head that provides inks of three or more different densities for a plurality of colors. In this case, nozzles corresponding to the respective colors of an equivalent density are included in an identical nozzle group.
In the printing apparatus of the present invention, the instruction of changing the dot creation timing is input for each nozzle group so as to adjust the dot creation timing. The technique of the present invention enables the dot recording positions of the respective nozzle groups to be more adequately aligned than the prior art technique. This arrangement effectively reduces the misalignment of dots created by different nozzle groups corresponding to, for example, different colors, thereby attaining the high-quality printing.
In conventional printing apparatus, the misalignment of dots between different nozzle groups may be found even at the time of shipment. Such misalignment is caused by the varying speed of ink election from the respective nozzle groups, which is ascribed to the respective nozzle groups on the print head having different ink ejection properties or the respective color inks having different properties.
The inventors of the present invention have found that the misalignment of dots occurring in the conventional printing apparatus is caused by a variety of factors arising after the shipment of the printing apparatus as discussed below in addition to the above factors. The ink ejection speed is generally affected by the viscosity of ink. It is practically impossible to make all the inks in any replaceable ink cartridges have a strictly identical viscosity. Namely, there is a variation in ink ejection speed among the respective inks. The viscosity of ink also varies with an elapse of service time and with a variation in temperature. The mechanism of ejecting ink is also subject to deterioration with age. In the conventional printing apparatus there is a misalignment of dot recording positions, due to a variation in ink ejection speed by the variety of factors arising after the shipment as well as those found even before the shipment.
The inventors of the present invention have developed the printing apparatus discussed above, based on the idea that the accurate adjustment of the dot creation timing of each nozzle group, not only during the manufacturing process of the printing apparatus but during its use, is the best way to cancel the misalignment due to the diversity of factors. The arrangement of the present invention enables the dot creation timing of each nozzle group to be accurately regulated in the manufacturing process of the printing apparatus. This arrangement also enables the user to adequately adjust the dot creation timing of each nozzle group after shipment. Even when a misalignment of the dot recording positions occurs after shipment due to any of the factors discussed above, the user can readily make the adequate adjustment, and thereby maintain the high picture quality of the resulting printed image.
One applicable method individually adjusts the dot creation timing with regard to each of the plurality of nozzle groups. Another applicable method fixes the dot creation timing with regard to a specific nozzle group and adjusts the dot creation timing with regard to another nozzle group relative to the fixed dot creation timing of the specific nozzle group.
As described previously, it is not necessary to regulate the dot creation timing for all the nozzle groups mounted on the print head. The nozzle groups having relatively small effects on the picture quality of the resulting printed image may be excluded from the objects of regulation of the dot creation timing.
This arrangement effectively reduces the misalignment of the dot recording position with regard to the nozzle groups having the significant effects on the picture quality of the resulting printed image, while excluding the individual adjustment of the dot creation timing with regard to the nozzle groups having relatively small effects on the picture quality. The technique thus ensures the significant improvement in picture quality of the resulting printed image, while reducing the labor required for the adjustment. The adjusted dot creation timing for any nozzle group or a preset fixed dot creation timing may be set to the dot creation timing with regard to each nozzle group having relatively small effects on the picture quality.
In the case of defining the nozzle groups by the colors, the nozzle groups having relatively small effects on the picture quality correspond to the colors of low visual conspicuousness and the colors of low density. In the printing apparatus with cyan, magenta, yellow and black inks, yellow is the color having the small effect on the picture quality. In the printing apparatus with inks of different densities, for example, cyan, light cyan, magenta, light magenta, yellow and black inks, the three colors (i.e., yellow, light cyan, and light magenta) are the colors having the small effect on the picture quality. In any case, such colors are adequately selected by considering the actual effects of the misaligned dots on the picture quality. The nozzle group having the small effect on the picture quality is not necessarily the nozzle group corresponding to the color of low density. In some images, the nozzle group having the small effect on the picture quality is the nozzle group used for dot creation with low frequency.
In the printing apparatus of the present invention, the adjustment of the dot creation timing may include software. However, it is preferable that the adjustment unit has a delay circuit corresponding to each of the plurality of nozzle groups. Here the delay circuit functions to adjust an output timing of a driving signal of the print head in response to the changing instruction.
The delay circuit may delay the output timing of the driving signal to the print head according to the number of pulses input into a counter circuit. A plurality of circuits having different output timings of the driving signal may be selectively used in response to the changing instruction. A variety of other structures may be applied for the delay circuit. The use of such circuits having relatively simple structures enables the accurate adjustment of the dot creation timing.
As mentioned previously, the misalignment of dot recording positions also occurs in the case of unidirectional recording. The technique of the present invention is thus applied to the printing apparatus of unidirectional recording that records dots only in either the forward pass or the backward pass of the main scan to improve the picture quality of the resulting printed image. The technique of the present invention is more preferably applied to the printing apparatus that drives the print head in both the forward pass and the backward pass of the main scan (i.e., the printing apparatus that performs the bidirectional recording).
Application of the technique of the present invention to the printing apparatus of bidirectional recording ensures the high-speed and high-quality printing of images. In the case of bidirectional printing, there is a fair possibility that the misalignment, which generally leads to the low picture quality, occurs not only between the dots created by the different nozzle groups but between the dots created in the forward pass and in the backward pass. The technique of the present invention is thus favorably applied to the printing apparatus of bidirectional recording to prevent such misalignment of dots and significantly improve the picture quality of the resulting printed image.
In the printing apparatus of bidirectional printing, the adjustment unit may be used to adjust the dot creation timing between the plurality of nozzle groups. The adjustment unit is, however, more preferably used to adjust the dot creation timing with regard to each nozzle group in each direction of the main scan. Namely, it is preferable the adjustment unit individually adjusts the dot creation timings in the forward pass and the backward pass of the main scan with regard to each nozzle group. This arrangement enables the dot recording positions to be aligned with a higher accuracy.
In accordance with one preferable application of the present invention, the printing apparatus of unidirectional recording includes a test pattern printing unit that prints a predetermined test pattern with each of the at least two nozzle groups that are objects of the changing instruction. The predetermined test pattern is set to allow detection of a relative misalignment of dots created in either the forward pass or the backward pass of the main scan.
In a similar manner, it is preferable that the printing apparatus of bidirectional recording includes a test pattern printing unit that prints a predetermined test pattern with each of the at least two nozzle groups that are objects of the changing instruction. The predetermined test pattern is set to allow detection of a relative misalignment of dots created in the forward pass of the main scan with dots created in the backward pass of the main scan.
There is a misalignment of dots created in the forward pass with dots created in the backward pass by each nozzle group. There is another misalignment of dots between different nozzle groups. The printing apparatus of the above application detects any combination of such misalignments of dot recording positions. This accordingly enables the accurate adjustment of the dot recording positions and thereby improves the picture quality of the resulting printed image.
It is not necessary the test pattern printing unit uses only one test pattern to detect all the misalignments of dot recording positions in the forward pass and the backward pass with regard to the respective colors. One applicable method successively prints a plurality of test patterns to detect such misalignments. It is also not necessary to detect all the misalignments in the forward pass and the backward pass with regard to the respective nozzle groups.
In addition, the test pattern printing unit may print the test pattern in any of various applications discussed below.
In accordance with a first preferable application of the printing apparatus, the test pattern printing unit includes a specific group test pattern printing unit that prints a first test pattern with one specific nozzle group, which is selected among the at least two nozzle groups that are the objects of the changing instruction. The first test pattern includes both dots created in the forward pass of the main scan and dots created in the backward pass of the main scan at a preset positional relationship. Also included is another group test pattern printing unit that prints a second test pattern. The second test pattern includes both dots created by another nozzle group other than the specific nozzle group and dots created by the specific nozzle group, to allow detection of a relative misalignment of dot recording positions of the another nozzle group and the specific nozzle group in at least one of the forward pass and the backward pass of the main scan.
This arrangement enables the dot creation timing to be adjusted with regard to the specific nozzle group to reduce the misalignment of dots created in the forward pass with dots created in the backward pass. The dot creation timing with regard to another nozzle group is adjusted relative to the dot creation timing of the specific nozzle group to reduce the misalignment of dots created in either the forward pass or the backward pass of the main scan. The dot creation timings in the forward pass and the backward pass of the main scan are adjusted with regard to each of the other nozzle groups by taking advantage of the adjusted dot creation timings in the forward pass and the backward pass with regard to the specific nozzle group. The test pattern printing unit of this arrangement enables the positions of dots created in the forward pass and the backward pass to be aligned with regard to the plurality of nozzle groups.
In accordance with a second preferable application of the printing apparatus, the test pattern printing unit prints a certain test pattern with each of the at least two nozzle groups that are the objects of the changing instruction. The certain test pattern includes both dots created in the forward pass of the main scan and dots created in the backward pass of the main scan at a predetermined positional relationship.
This arrangement enables the positions of dots created in the forward pass of the main scan to be aligned with dots created in the backward pass of the main scan with regard to each nozzle group. This technique is preferably adopted to adjust the dot creation timing, in the case where there is a more significant misalignment between dots created in the forward pass and in the backward pass than a misalignment between dots created by different nozzle groups. The technique thereby readily improves the picture quality of the resulting printed image.
In accordance with a third preferable application of the printing apparatus, the test pattern printing unit prints a specific test pattern with a certain nozzle group corresponding to a specific color of low visual conspicuousness, among the at least two nozzle groups that are the objects of the changing instruction. The specific test pattern includes both dots created by the certain nozzle group and dots created by another nozzle group, which has an adjusted dot creation timing, to enhance visual conspicuousness of the relative misalignment.
This arrangement ensures the accurate adjustment of the dot creation timing with regard to the dots of low visual conspicuousness. For example, it is assumed that yellow dots are printed on white printing paper. Since the yellow dots have low visual conspicuousness, it is difficult to adjust the dot recording positions with high accuracy. In such cases, the yellow dots are formed with the cyan dots having the adjusted dot creation timing in an overlapping manner to be expressed as green dots. This technique enhances the visual conspicuousness and ensures the more accurate adjustment of the dot creation timing. Dots of different colors may be mixed in a variety of patterns.
In the printing apparatus having the test pattern printing unit, it is desirable that the timing specification unit specifies the dot creation timing based on a relation to the printed test pattern.
This arrangement facilitates the specification of the dot creation timing. One procedure allocates preset indexes to respective dot creation timings, at which the test pattern is printed, and specifies the adequate dot creation timing by the index. Another procedure repeats the cycle of printing the test pattern at a selected dot creation timing and determines whether or not the selected dot creation timing is adequate, thereby specifying the appropriate dot creation timing.
In accordance with another preferable application of the printing apparatus, the adjustment unit may define the dot creation timing of a specific nozzle group as a standard and advance or delay the dot creation timing of another nozzle group relative to the standard. In this case, to allow printing at an earlier dot creation timing than the standard dot creation timing, it is desirable to set the standard to a specific dot creation timing for creating dots with delay of a predetermined time period since the input of a signal indicating dot creation to the print head. This arrangement enables printing at an earlier dot creation timing than the standard dot creation timing in the range of the predetermined delayed time period.
In another embodiment, the adjustment unit may define a specific nozzle group having an earliest dot creation timing, among the at least two nozzle groups that are the objects of the changing instruction, as a standard and adjusts the dot creation timing of another nozzle group relative to the standard.
This arrangement enables the adjusted dot creation timing of each nozzle group to have certain delay relative to the standard dot creation timing. This arrangement is free from the undesirable restriction as in the case of fixing a specific color to the standard, but enables the dot creation timing to be adjusted in a wider range.
The technique of the present invention is also actualized in the form of a recording medium, in which a specific program is recorded as discussed below.
The present invention is accordingly directed to a first recording medium in which a specific program is recorded in a computer readable manner. The specific program functions to adjust a dot creation timing with regard to each of a plurality of nozzle groups in a printing apparatus that performs a main scan and causes a print head having nozzles for ejecting ink to create dots on surface of a printing medium at a predetermined dot creation timing in the course of the main scan. Each of the nozzle groups includes a plurality of nozzles having a predetermined common condition relating to ink ejection and the main scan moves the print head forward and backward relative to the printing medium. The specific program causes a computer to attain functions of printing a predetermined test pattern with each of at least two nozzle groups selected among the plurality of nozzle groups (the predetermined test pattern being set to allow detection of a relative misalignment of dots created in the forward pass of the main scan with dots created in the backward pass of the main scan), inputting a specified dot creation timing with regard to each of the nozzle groups, based on a relation to the printed test pattern, and changing a parameter, which specifies the dot creation timing, with regard to the each nozzle group based on the specified dot creation timing.
The present invention is also directed to a second recording medium in which a specific program for driving the printing apparatus of the present invention is recorded in a computer readable manner. The specific program causes a computer to attain a function of adjusting the dot creation timing with regard to each color in response to the changing instruction.
The computer executes the program recorded in either one of these recording media, so as to perform the adjustment of dot creation with regard to a plurality of nozzle groups. Typical examples of the recording medium include flexible disks, CD-ROMS, magneto-optic discs, IC cards, ROM cartridges, punched cards, prints with barcodes or other codes printed thereon, internal storage devices (memories like a RAM and a ROM) and external storage devices of the computer, and a variety of other computer readable media. The present invention is also directed to the program itself or a diversity of equivalent signals.